Non-invasive imaging and analysis of targets is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process. In the case of quality control, it enables non-destructive imaging and analysis on a routine basis.
Optical coherence tomography (OCT) is a technology for non-invasive imaging and analysis. There are more than one OCT techniques. Time Domain OCT (TD-OCT) typically uses a broadband optical source with a short coherence length, such as a super-luminescent diode (SLD), to probe and analyze or image a target. Multiple Reference OCT (MRO) is a version of TD-OCT that uses multiple reference signals. Another OCT technique is Fourier Domain OCT (FD-OCT). A version of Fourier Domain OCT, called Swept Source OCT (SS-OCT), typically uses a narrow band laser optical source whose frequency (or wavelength) is swept (or varied) over a broad wavelength range. In TD-OCT systems the bandwidth of the broadband optical source determines the depth resolution. In SS-OCT systems depth the wavelength range over which the optical source is swept determines the depth resolution.
TD-OCT technology operates by applying probe radiation from the optical source to the target and interferometrically combining back-scattered probe radiation from the target with reference radiation also derived from the optical source. The typical TD-OCT technique involves splitting the output beam into probe and reference beams, typically by means of a beam-splitter, such as a pellicle, a beam-splitter cube or a fiber coupler. The probe beam is applied to the system to be analyzed (the target). Light or radiation is scattered by the target, some of which is back scattered to form a back-scattered probe beam, herein referred to as signal radiation.
The reference beam is typically reflected back to the beam-splitter by a mirror. Light scattered back from the target is combined with the reference beam, also referred to as reference radiation, by the beam-splitter to form co-propagating reference radiation and signal radiation. Because of the short coherence length only light that is scattered from a depth within the target whose optical path length is substantially equal to the path length to the reference mirror can generate a meaningful interferometric signal.
Thus the interferometric signal provides a measurement of scattering properties at a particular depth within the target. In a conventional TD-OCT system, a measurement of the scattering values at various depths can be determined by varying the magnitude of the reference path length, typically by moving the reference mirror. In this manner the scattering value as a function of depth can be determined, i.e. the target can be scanned.
There are various techniques for varying the magnitude of the reference path length. Fiber based systems use fiber stretchers, however, they have speed limitations and have size and polarization issues. Rotating diffraction gratings can run at higher speeds, however, such gratings are alignment sensitive and have size issues.
Piezo devices can achieve high speed scanning and can have high pointing accuracy, however to achieve a large scanning range requires expensive controls systems and have limited speed. A scanning method that effectively amplifies the scan range of a piezo device is described in the U.S. Pat. Nos. 7,526,329 and 7,751,862 incorporated herein. This scanning method is also applicable to electro-mechanical voice coil actuators that can have considerable scanning range.
The technique described in these incorporated references uses multiple reference signals with increasing scan range and correspondingly increasing frequency interference signals. This scanning method can achieve large scan range at high speed with good pointing stability.
The interference signals associated with the multiple references are detected by a single detector as a complex signal consisting the combined interference signals and noise of various types. Types of noise may include; optical noise in the optical source; unwanted interference signals due to reflections from surfaces of optical components; detector noise; shot noise of a photo-diode; and electronic noise.
In swept source Fourier domain OCT systems depth scanning is accomplished by repeatedly sweeping the wavelength of the optical source. The wavelength range over which the optical source is swept determines the depth resolution. The period of the sweep repetition rate determines the period of the depth scans.
In addition to depth scanning, lateral scanning is required for many imaging and analysis applications. There are many conventional techniques for lateral scanning, such as the use of stepper or linear motors. Some applications require angular scanning, which is typically accomplished by electro-mechanical oscillating mirrors, typically referred to as galvo-scanners.
In conventional TD-OCT systems the detected interference signals typically are centered about a specific frequency that is determined by the speed with which the reference path length varies and the center wavelength of the light being used, while in SS-OCT systems the detected interference signals typically are centered about a specific frequency that is determined by the speed with which the optical source is swept and the wavelength of the light being used. The interference signals are typically filtered and down converted to a baseband signal prior to digitization and further processing.
In Multiple Reference OCT (MRO), a version of TD-OCT that uses multiple reference signals, the set of detected interference signals are centered about a frequency that is determined by the speed with which the reference path length varies and the center wavelength of the light being used and integer multiples of this frequency.
An MRO system using a piezo devices as a scanning method can be operated by driving the piezo device with a constant speed for a significant portion of its scan range, referred to as the linear range. In this case the detected interference signals have constant frequencies over the linear range, i.e. the signals are linearized by ensuring the piezo is driven at constant speed.
Alternatively the MRO system can be operated by driving the piezo device with a sinusoidal (or other) waveform. In this case the detected interference signals have frequencies that vary continuously over the range of the piezo scan. The resulting digitized detected signals are typically post-processed to compensate for the varying or non-linear motion of the piezo device. The resulting “linearized” signals have constant frequencies over a significant portion of the scan range.
In both of the above MRO systems the constant frequency signals or the linearized signals are further processed by using a bank of filters with center frequencies at integer multiples of a fundamental frequency in order to separate out the information contained at the different frequencies and relating to different scan segments. This is described in more detail in the U.S. Pat. Nos. 7,526,329 and 7,751,862 incorporated herein by reference.
This processing approach has several disadvantages that negatively impact the signal to noise ratio (SNR) that the MRO system can achieve. For example: linearizing the interference signals by driving the piezo at constant speed requires the drive signal be matched to the specific piezo device (whose characteristics may change with time) and precludes using low cost transformers for voltage amplification; linearizing the interference signals by post-processing the digitized data distorts the noise characteristics of the system; crosstalk can occur between different frequencies, an issue that is exacerbated as the bandwidth of the optical source is increased; the information from different scan segments must be normalized in amplitude and aligned with each other; the SNR decreases for the higher order reference signals.
There is therefore an unmet need for a method, apparatus and system for acquiring and processing interference signals for optical coherence tomography systems, and similar such non-invasive imaging and analysis techniques and systems, that enables using low cost transformers for voltage amplification; that does not distort the noise characteristics of the system; that eliminates crosstalk especially as the bandwidth of the optical source is increased; that does not require scan segment amplitude normalization or alignment, and that maintains good SNR even for the higher order reference signals.